Currently, bone defects are typically repaired by autografts or banked bone. Autografts have a good ability to unify the bone, and physicians often prefer to use bone from sources such as the iliac crest. However, procedures using autografts suffer from several drawbacks. First, autografts require a separate harvest operation, resulting in increased operative time and the use of blood transfusions. Secondly, patients often lack adequate amounts of material for harvesting and often experience donation site morbidity. Implantation of banked bone does not require the harvest operation, but its bone healing capability is not as high as that of autografts. Therefore, it is undesirable to use banked bone in severe conditions such as nonunion.
Because of these drawbacks, researchers have searched for compositions and methods for promoting bone growth without necessitating the use of autografts or banked bones. One potential source for bone growth promoting factors is the extracellular matrices of healthy bone and cartilage tissues. Extracellular bone matrix contains predominantly mineral (hydroxyapatite) and an organic matrix, where the major component of the organic matrix is collagen type I. The remaining components of bone matrices include a number of less abundant non-collagenous proteins and growth factors. For example, since the mid-1960's the osteoinductive activity of both demineralized bone matrix (DBM) and bone morphogenetic protein (BMP) has been studied, e.g., Ijiri (1992). In addition to DBM and BMP, many compounds possess biological activity and find wide use in medical applications such as prosthetic devices, drugs, blood components, and the like.
Endothelins, are a class of 21-amino acid vasoactive peptides with intra chain disulfide bonds. The proteins were initially isolated and purified from conditioned medium of cultures of porcine aortic endothelial cells. Endothelins have subsequently been found to be produced by many cells, and their production modulated by systemic hormones and local factors (Rubanyi and Polokoff, 1994). Endothelin can be isolated from a wide variety of warm-blooded vertebrate tissue sources using standard extraction techniques known to those skilled in the art.
It has been reported that incubation of cell populations with the endothelial cell polypeptide endothelin-1 affects osteoblast growth and function. Accordingly, there has been speculation that these proteins as well as numerous other isolated compounds may play a role in bone and cartilage repair. However, in vitro results may vary due to a variety of factors, including cell type, cell density, cell isolation procedures, and type of growth medium. Therefore, while useful, in vitro studies are not always predictive of in vivo activity.
For example over the last two decades prostaglandins had been reported as both increasing bone resorption as well as increasing bone formation. Analysis of the literature references reporting the conflicting activities of prostaglandins reveals that almost all reports of bone resorption were performed in vitro and almost all the studies reporting bone formation were done in vivo (Mark and Miller, 1993). The studies of bone growth in vitro were performed with tissue/organ cultures of bone or relatively pure isolated bone cell populations. The apparent conflicting reports of the predominant skeletal affects of the prostaglandins can be explained on the basis of the limitations of the cell culture systems used to study those effects. Similarly, initial reports of TGF- activity based on cell culture assays failed to correlate with observed in vivo activities. Therefore, skilled artisans appreciate that in vitro activity does not always predict in vivo results.
What is needed are compositions shown to repair connective tissue in vivo.